I have some data to display in a facetted levelplot (contour lines), and was very surprised to see strange noisy contour lines in one of the panels where the data were basically constant. I set the binwidth to 1 for the whole layer (all panels), and the subset of the data for this panel was in the range 1 +/- 1e-10, so I expected to see no contour line there. After much subsettting, here is an example of data that shows a contour line, where I wouldn't want/expect to see one.